The developmental mechanisms responsible for formation of neuronal connections in the vertebrate nervous system remain largely unknown. The goal of my research is to elucidate these mechanisms. In these experiments I will analyze the development of connections between sensory afferents and motoneurons in the spinal cord of the chick embryo. The great experimental convenience of this system makes it possible to test several alternative mechanisms that could account for the formation of specific connections between sensory and motor neurons. I will analyze two aspects of the connectivity between afferents and motoneurons: the specific connections between particular muscles and individual motoneuron pools; and the strength of connections between particular muscles and different spinal segments. Connections will be measured by monosynaptic reflex testing and the recording of synaptic potentials in the ventral roots and individual muscle nerves. First, I will test the hypothesis that cell death eliminates erroneous connections between afferents and motoneurons. This will be achieved by preventing sensory and motoneuronal cell death and establishing if this results in abnormal sensory-motor connections. Secondly, I will establish the importance of positional factors in controlling the sensory connections between muscles and the different spinal segments, by comparing the connectivity between afferents and either normally or ectopically located motoneurons. Finally, I will use reversals of the neural crest to test the hypothesis that sensory cells are specified early in development to innervate particular peripheral and central targets.